In recent decades, the air transport brings great convenience in travel, but simultaneously also poses a serious noise pollution around the airport. According to the investigation, the airplane noise seriously occurs upon taking off or landing, especially upon taking off. The airport is usually near to the city, more over the flight course is crossing the urban sky partially. Therefore the aircraft noise has become an important component of the urban noise, which influenced the people's normal life seriously, and it's urgent and imminent problem how to reduce the airplane flight noise maximally, especially when taking off or landing.
The aviation noise mainly results from the jet and fan of the aircraft engine. Over the years, researchers have been committing to find out an acoustic attenuation for reducing jet noise and fan noise with a good sound absorption effect over a wider range of frequency, and at present, generally the following methods and design criteria are adopted:                1. Designing the engines and nozzle with high bypass ratio to reduce jet noise;        2. Adjusting the design and speed of the fan rotor/stator blade to reduce fan noise; and        3. Applying the acoustic lining to the internal surface of the pipeline to reduce radiated engine noise.        
Although laying the acoustic lining in pipe surface of the engine has achieved a good effect in the aspect of reducing noise, it still can't meet the present real requirement of the aircraft noise control, the environmental noise control around airports and the noise airworthiness. It is reported that Airbus is investing a lot of money to participate in the aircraft noise research project focusing to overcome the airplane noise reduction technology initiated by the nine European states, with a target to reduce the noise of airplane produced by it in 6 decibels on the basis of the present noise within 8 years. In fact, the mean noise reduction value each 10 years is less than 8 dB with respect to the mean noise reduction amplitude of the aircraft noise.
Application of the prior methods and means is unable to achieve a substantial noise reduction of aircraft engines, because the middle- or low-frequency noise usually can't be effectively controlled in the environment of the high acoustic intensity, resulting in having to substantially increase the depth of the air cavity of the sound-absorbing structure for enhancing its absorption, which usually can't be completed in the actual noise reduction. Therefore, it is necessary to further develop of the capability of absorbing sound for the prior the lining of the surface of the prior aerial engine, for the basic structure does not change the existing muffler case to explore further the method and measures for improving the capability of absorbing the sound on the surface of the prior aircraft engine endeavoring not to change the structure of the prior muffler.
Hereafter, we obtain the present invention in the teaching of the impact on the acoustic transmission in the atmosphere by the nature temperature gradient. During the day, we may feel the environment around us more noisy than at night, which is resulted from the great impact of the temperature gradient on the transmission of the acoustic wave by the way that the sonic speed is closely related with the temperature, and the temperature gradient causes the sonic speed to change above the ground. During the day, specially on a sunny day, in the afternoon with sunlight, a significant negative temperature gradient from the ground results in the surface temperature is higher than the air temperature, the speed of the sonic speed on the ground is faster, and the sonic speed on the sky is slower, so that the sound may transmit along the path taking the least time according to the Fermat's principle. Thus, during the day, the acoustic ray may bend to and refract toward “the cold end—Sky”, and the acoustic ray refracted upward almost doesn't contact the earth which can not play the role as a “sound-absorbing structure”, notwithstanding the simultaneous emergence of “the acoustical shadow zone”. Therefore, during the day we will feel the environment around us more noisy. In contrast, at night, the positive temperature gradient above the ground results in the surface temperature is lower than the atmospheric temperature, the sonic speed on the ground is slower, the sonic speed on the sky is faster, so that the acoustic ray will bend to and refracted toward “the cold end”, the earth, according to the Fermat's principle, and only part of the acoustic wave is reflected back to the atmosphere, but most of the energy is transmitted through the pores of the soil, absorbed and attenuated by the earth for the incidence of the acoustic wave of the air to the porous earth. It is under the effect of the temperature gradient using the earth as code end at night that the earth fully functioned as “sound-absorbing structure”, resulting in feeling the silence of environment around us at night.
In addition, in the case of snowing, we can feel the quietness resulting from the earth as “sound-absorbing structure covered with the snow” with the effect of temperature gradient using the earth as cold end. The FIGS. 2 and 3 show the deflection of the acoustic ray in the case that temperature decreases with the height during the day and the temperatures increases with the height.
Based on the example of the impact of the temperature gradient in the nature has on the atmospheric sound transmittance, we can come to the conclusion that the acoustic ray will deflect toward the cold end in the effect of the temperature gradient, making it possible to capture and attenuate the noise using the “cold trap”, and further to develop a muffler having a low temperature refrigeration system based on the temperature gradient and the acoustic ray deflection by the above method, so as to further improve the ability of the conventional muffler to attenuate the noise, for the present invention.
However, by far, the research in the design of the muffler only considered the influence of temperature, without taking into account the impact of the acoustic ray reflection the use of temperature gradient exist within the muffler leading to voice deflection for improving the muffler volume effects, such as literature: “Perforated pipe muffler Numerical Analysis” Helie Zhu, Zhengshi Liu, Journal of noise and vibration control in June 2008 (3); “Engine Muffler Static Performance Testing and Analysis,” Xiaojun Wang, Zhiyuan Li, Haozheng Wei, Journal of noise and vibration control, 2007 4th issue.